1. Field of the Invention
The present invention relates to a nitride-based semiconductor element and a method of forming a nitride-based semiconductor, and more specifically, it relates to a nitride-based semiconductor element including a nitride-based semiconductor layer formed by epitaxial lateral overgrowth and a method of forming a nitride-based semiconductor.
2. Description of the Prior Art
In recent years, a nitride-based semiconductor element utilizing a group III nitride-based semiconductor is actively developed as a semiconductor element employed for a semiconductor light-emitting device such as a light-emitting diode device or a semiconductor laser device or an electronic device such as a transistor. In order to fabricate such a nitride-based semiconductor element, a nitride-based semiconductor layer is epitaxially grown on a substrate consisting of sapphire or the like.
In this case, the substrate of sapphire or the like and the nitride-based semiconductor layer have different lattice constants and hence the nitride-based semiconductor layer grown on the substrate of sapphire or the like has dislocations (lattice defects) vertically extending from the substrate to the surface of the semiconductor layer. Such dislocations in the nitride-based semiconductor layer result in deterioration of the element characteristics of the semiconductor element and reduction of the reliability thereof.
As a method of reducing the density of the aforementioned dislocations in the nitride-based semiconductor layer, epitaxial lateral growth is generally proposed. This epitaxial lateral growth is disclosed in International Workshop on Nitride Semiconductors -IWN2000-, Nagoya, Japan, 2000, p. 79, for example.
FIGS. 29 to 33 are sectional views for illustrating a conventional method of forming a nitride-based semiconductor employing epitaxial lateral overgrowth. The conventional method of forming a nitride-based semiconductor employing epitaxial lateral overgrowth is now described with reference to FIGS. 29 to 33.
First, a GaN layer 102 for serving as an underlayer is formed on a substrate 101 consisting of sapphire or SiC, as shown in FIG. 29. Then, mask layers 103 are formed on prescribed regions of the GaN layer 102.
Then, portions of the GaN layer 102 located under regions formed with no mask layers 103 are removed by etching while etching the substrate 101 by a thickness in the range not reaching the bottom surface thereof through the mask layers 103 serving for etching in this process. Thus, the substrate 101 is brought into a ridged shape, while stripe-patterned GaN layers 102 to be in contact substantially with the overall upper surfaces of projection potions of the substrate 101, as shown in FIG. 30.
Then, undoped GaN layers 104 are re-grown from exposed side surfaces, serving as seed crystals, of the GaN layers 102, as shown in FIG. 31. The undoped GaN layers 104 are laterally grown in an initial stage. From the state shown in FIG. 31, the undoped GaN layers 104 are grown upward while laterally growing on the mask layers 103 serving for selective growth in this process, as shown in FIG. 32. At this time, voids 105 are formed between the undoped GaN layers 104 and the bottom surfaces of recess portions of the substrate 101. The undoped GaN layers 104 laterally growing on the mask layers 103 coalesce into a continuous undoped GaN layer 104 having a flattened surface, as shown in FIG. 33.
In the conventional method of forming a nitride-based semiconductor, as hereinabove described, the undoped GaN layer 102 is formed by epitaxial lateral overgrowth from the exposed side surfaces of the GaN layers 102 serving as seed crystals, whereby lattice defects are scarcely propagated from the GaN layers 102 to a portion around the surface of the undoped GaN layer 104. Thus, the undoped GaN layer 104 reduced in dislocation density is obtained. When a nitride-based semiconductor element layer (not shown) having an element region is formed on such an undoped GaN layer 104 reduced in dislocation density, a nitride-based semiconductor element having excellent crystallinity can be formed.
In the aforementioned conventional method of forming a nitride-based semiconductor employing epitaxial lateral overgrowth, however, the substrate 101 is brought into the ridged shape by removing the portions of the GaN layers 102 located under the regions formed with no mask layers 103 by etching and thereafter further etching the substrate 101. In general, therefore, the layers GaN 102, which are hardly etched nitride-based semiconductor layers, must be etched along the overall thicknesses thereof while the surface of the substrate 101 must also be etched. Thus, the etching time for bringing the substrate 101 into the ridged shape is disadvantageously increased. Consequently, the nitride-based semiconductor is disadvantageously reduced in mass productivity.
In the aforementioned conventional method of forming a nitride-based semiconductor employing epitaxial lateral overgrowth, further, the undoped GaN layer 104 is formed by growing the GaN layers 102 serving as underlayers on the substrate 101 and thereafter epitaxially laterally overgrowing the GaN layers 102. Therefore, this method requires two crystal growth steps for the GaN layers 102 and the undoped GaN layer 104. In general, therefore, the nitride-based semiconductor is reduced in mass productivity also in this point.